Conventionally, an image forming apparatus for developing an electrostatic latent image on the surface of an image supporting member with a liquid developer including toner particles dispersed in an insulating carrier liquid, transferring a toner image formed by development onto a recording material, thereby obtaining a final image is known. Further, an image forming apparatus for transferring primarily a toner image formed on an image supporting member by development with a liquid developer onto an intermediate transfer medium, superimposing toner images of a plurality of colors on the surface of the intermediate transfer material, thereafter transferring the superimposed toner images onto a recording material in a batch, thereby obtaining a final color image is known.
A wet type image forming apparatus using a liquid developing device has advantages which cannot be realized by a dry type image forming apparatus, and in recent years, the value has been reconsidered. The main advantage of the wet type image forming apparatus is that very fine toner of a submicron size can be used, so a high image quality can be realized, and a texture equivalent to printing can be obtained. Particularly, in recent years, in correspondence to speed-up of the image forming apparatus, there has been a trend to use a liquid developer including toner particles dispersed in high concentration in a carrier liquid of high viscosity.
For transfer of a toner image from an image supporting member or an intermediate transfer medium to a recording material in the wet type image forming apparatus, generally, an electrostatic transfer system by electrostatic force is used. Toner particles are charged, so when a voltage with the reverse polarity to the charging polarity of the toner particles is impressed to the transfer roller installed on the rear side of the recording material, the toner particles move to the surface of the recording material by the electrostatic force.
In such an image forming apparatus, to output an image of a high quality regardless of the kind of a recording material used, it is necessary to execute transfer to the recording material stably and highly efficiently.
Japanese Laid-Open Patent Publication H9-304979 discloses the image forming apparatus for controlling, depending on the characteristics of transfer sheets, the toner adhesion amount (the potential of the photoconductor, developing bias voltage), bias voltage to the set roller for stabilizing the toner adhesion condition on the photoconductor, charging current to the photoconductor, transfer current, and fixing temperature. However, the art recorded in this document does not adjust the charging quantity given to toner depending on the property of the recording material, so an image of a sufficiently high quality cannot be outputted.
Further, U.S. Pat. No. 6,115,576 discloses the image forming apparatus for giving a charge of the same polarity as that of toner to a toner image on an intermediate transfer belt. However, the art recorded in this document does not carry out control depending on the property of the recording material, so the transfer efficiency of toner onto the recording material may be deteriorated extremely.
Further, U.S. Pat. No. 6,766,123 discloses the dry type image forming apparatus which has a pre-transfer charging roller, prior to transfer of a toner image, for charging the surface of a recording material with reverse polarity to the charging polarity of the toner and changes a voltage impressed to the charging roller depending on the kind of the recording material. However, the art recorded in this document does not adjust the charging quantity of toner depending on the property of the recording material either, thereby cannot accomplish an object of outputting an image of a sufficiently high quality.